What is the equation of the separation line for this neuron with identity activation?
I found the answer, the output u is not related to plotting x2 against x1 here, coz u is like z axis and seeing the separation line is looking perpendicularly to the x1x2 plane, and whatever u value is, it's just a dot.

Put u as zero for the solving steps in the question.

So the separation line equation is this (with any activation function):

$x_2 = \dfrac{-w_1}{w_2}x_1 + \dfrac{-b}{w_2}$